In hydraulic systems wherein hydraulic fluid from a variable displacement pump or other source of pressurized hydraulic fluid is supplied to one or more hydraulic actuators, such as a double acting hydraulic piston actuator, and wherein the desired flow rate of pressurized hydraulic fluid is determined by the total flow required instantaneously to all activated actuators, various systems have been utilized to sense the fluid pressure in the hydraulic actuator as an indication of the load demand of the actuator and to transmit such fluid pressure as a control signal to a device for controlling the discharge rate from the source of pressurized fluid.
In order for the load demand control signal to follow decreases in fluid pressure in the actuator, representing drops in load demand, it is necessary to bleed the fluid pressure of the control signal to a reservoir. The bleed rate must be sufficiently rapid to permit the desired responsiveness of the control ,signal. However, when the hydraulic actuators have been inactive for even a short period of time, the control signal can drop all the way to the reservoir pressure, with some or all of the hydraulic fluid in the load demand control signal line having drained through the bleed restriction into the reservoir. Thereafter, when one of the hydraulic actuators is activated, the load demand pressure signal lags behind the pressure of the hydraulic fluid going to the actuator. This lag is the result of the time required to refill the load demand signal line and bring the pressure therein up to the value corresponding to the load demand.
Such systems are utilized in various types of mobile equipment such as backhoes, loaders, agricultural tractors, road graders, etc., wherein the operator manually moves actuating levers to control hydraulic functions, such as power steering, raising or lowering of attachments, and the like. When the operator moves one of the actuating levers, he expects an immediate response with the desired movement of the function he was trying to adjust. However, the output of the pressurized fluid source is not sufficient to provide the increased hydraulic fluid demand, because the load sense pressure signal has not yet activated the device that controls the discharge rate of the pressurized fluid source to increase the discharge rate as needed to meet the new demand. The delay in actuation of the discharge rate control device is due to the lag in the load sense pressure signal, caused by prior partial draining of the load sense line. This delay in the response of the function which the operator is trying to adjust makes it difficult for the operator to control the function accurately. This is particularly acute when the operator is attempting multiple operations simultaneously, such as varying the angle and elevation of a blade on a road grader or lowering the boom and actuating the scoop on a backhoe.
The system response of the hydraulic system is affected by the type of conduit used for the load sense line, e.g., hose or tubing, the inside diameter of the line, and oil compressibility. However, changes in these parameters did not overcome the lag problem and achieve satisfactory results. One way to hold a residual pressure in the load sense line would be to employ a plurality of orifices located in series; however, such residual pressure would be unstable and temperature sensitive.